Sun is the only source of energy for all ecosystems on Earth. Out of which, less than 50% of the total incident solar radiation is useful for photosynthesis, which is called as Photosynthetically Active Radiation (PAR). Plants capture only 2-10 per cent of the PAR for photosynthesis.

Biomass: Mass of living organisms. Biomass of species is expressed in terms of fresh or dry weight.

Standing Crop: Mass of living material at each trophic level at a particular time. The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.

Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis, i.e., light energy converted to chemical energy.It is expressed in terms of weight (g –2) or energy (kcal m–2).

Productivity: Rate of biomass production. It is expressed in terms of g–2 yr –1 or (kcal m–2) yr–1. Productivity can be divided into Gross Primary Productivity (GPP) and Net Primary Productivity (NPP).

Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis.